The invention relates to a charge-coupled device comprising a charge transport channel defined in a semiconductor body and an electrode system for controlling the transport of charges through the charge transport channel, a shift register being provided, by means of which such voltages can be applied to the electrode system that during the storage of charge packets the device is operated as an m-phase device and is operated during transport as an n-phase device (n being larger than m), while a charge packet does not participate in the transport until the preceding charge packet has already been displaced over a certain distance.
Such a device is known inter alia from the article "The Accordion Imager: An Ultra-High-Density Frame Transfer CCD" by A. J. P. Theuwissen et al, published in "Technical Digest IEDM" (1984), p. 40/43. In this publication an image sensor of the frame transfer type is described. During the integration period, the sensor section is operated as a 2-phase device, at every other electrode a potential well being induced in which photo-current is integrated. The intermediate electrodes act as barrier electrodes between the potential wells. During the transport the device is operated as a 4-phase device, while, in order to prevent packets of charge being joined together, first only the first image line is displaced until the distance from the second image line is sufficiently large, after which simultaneously the first and second image lines can be displaced over a certain distance, etc. The image lines are therefore not displaced simultaneously from the beginning of the transport, but each time a new image line is added to the transport when the distance from the preceding image line allows it. In a similar manner, the information in the memory section can be telescopically compressed, while at every other electrode again a charge packet is stored.
In order to apply to the electrodes successively clock voltages or a d.c. voltage, use is made of one or more shift registers. The outputs of the separate stages of the shift register can be directly connected to the electrodes, as is described, for example, in the aforementioned publication, or can be coupled to a series of switches, through which the electrodes can be connected either to clock lines or to d.c. lines, as is described, for example, in the Netherlands patent application No. 8301977, which corresponds to U.S. patent application Ser. No. 615,843.
In these known CCD's of the accordion type, the electrodes are in principle equivalent to each other, at least during the charge transport, that is to say that each electrode acts as a charge storage electrode and a clock voltage of, for example, 10 V is applied to each electrode. In given applications, more particularly with large CCD's, a clock system having a comparatively low dissipation may be desirable. An object of the invention is therefore to provide an accordion CCD of the kind described above, which can be operated so that the dissipation is lower than in the known accordion CCD's with unchanged storage capacity.
Due to the fact that moreover in the known CCD's each electrode is associated with a stage of a shift register, technical problems with respect to designing are liable to arise during the manufacture of the device because the pitch between the stages of the shift register and the pitch between the electrodes of the charge-coupled device generally do not correspond to each other. In general solutions have to be found which require a comparatively large amount of space, more particularly due to the wiring from the shift registers to the electrodes. A further object of the present invention is therefore to provide an accordion CCD with simpler designing rules than the known device.
In certain cases, more particularly in image sensors intended to convert radiation images into electrical signals, it may be desirable to reverse the transport direction, as described, for example, in the co-pending Netherlands Patent Application "CCD image sensor arrangement", which corresponds to U.S. patent application Ser. No. 282,848.
A further object of the invention is therefore to provide an accordion CCD, in which the transport direction can be reversed in a simple manner.